The latest version released is "Nokia Belle Feature Pack 2" (e.g., for the last Nokia Symbian device, the Nokia 808 PureView).

Whatever is the latest version for your phone model depends on which phone you have.

Except for those phones with Symbian^3, Anna or Belle, they cannot be upgraded to newer versions of Symbian (just newer firmware versions but where the actual Symbian OS version remains whatever it was when the phone was released).

Only those phones that came out with Symbian^3 could be upgraded to Anna, Belle, or those that came out with Anna can be upgraded to Belle.

Nokia Belle FP2 will probably be, more or less, the last Symbian version Nokia is ever going to release, as it seems that no more new Symbian based models are coming, and Nokia has fired or outsourced most Symbian developers already, and even the outsourcing partner, Accenture, has fired or moved most of the Symbian people to other projects already.

I would like to tell you that you can easily upgrade your mobile phone operating system from the Nokia's official website. It will help you to update your phone fully and you can see new changes on your phone and this os will run very smoothly than the old one.